Transcript
General Chemistry LaboratoryUniversity of Kentucky
Determining the Enthalpy of a
Chemical Reaction
Pre-Lab
Bring completed pre-lab to classUse net ionic equations
Include pre-lab results in introduction section of report
Results of pre-lab assignment are your accepted valuesUse only for determining percent error
Enthalpy & Heat
Enthalpy (H) - Amount of heat change occurring during reactionExothermic (heat released, DH < 0)Endothermic (heat absorbed, DH > 0)
Heat (q) - Amount of energy transferred
q=H under constant pressure
Calorimetry
Measurement of heat changes Performed in a calorimeter which
prevents heat loss from the systemConstant volume calorimeterConstant pressure calorimeter
Calorimeter
Used to prevent the escape of heat
One version - Styrofoam cup in a beaker with a cardboard lid
q and DH
qUnits of JoulesExtensive propertyValue only true for the experimental
amounts of reactants and products DH
Units of Joules/mole Intensive propertyValue based on the moles given by the
coefficients of balanced equation
Relationship of q and DH
WATCH the units!!! DH will be expressed as a negative
or positive dependant on the direction of heat flow Negative: exothermic, heat released Positive: endothermic, heat consumed
Hmolesq
D#
q = heat, units of Joules m = mass, units of grams s = specific heat of water, units of DT = change in temperature, units of
Initial temperature: Right before
solutions are mixedFinal temperature: Maximum
temperature in data
q = msDT
Finding Enthalpy
Mix reactants Measure temperature change Calculate heat change Calculate change in enthalpy (DH)
Experimental Results
Enthalpy values for Reactions 1, 2, and 3
Two experimental values for reaction 3Value from running the reactionUsing Hess’ law from reactions 1 & 2
Accepted values are those from pre-lab
Hess’s Law
Enthalpy of a reaction does not depend on number of steps involved
If the enthalpy of a set of reactions is known, use combination of reactions to find enthalpy
Hess’s Law : Example
DH (kJ/mole) C(gr) + O2(g) CO2(g), DH0
rxn1= -393.5 S(rh) + O2(g) SO2(g), DH0
rxn2= -296.4CS2(l) + 3O2(g) CO2(g) + 2SO2(g) DH0
rxn3= -1073.6
Does this add up to: C(gr) + 2S(rh) CS2(l)?
Determine the enthalpy of the reactionshown using the three reactions given.C(gr) + 2S(rh) CS2(l)
C(gr) + 2S(rh) CS2(l)
C(gr) + O2(g) CO2(g) S(rh) + O2(g) SO2(g) CS2(l) + 3O2(g) CO2(g) + 2SO2(g) Therefore, rewrite equations C(gr) + O2(g) CO2(g) 2S(rh) + 2O2(g) 2SO2(g) 2SO2(g) + CO2(g) CS2(l) + 3O2(g) C(gr) + 2S(rh) CS2(l)DH0
rxn = DH0rxn1 + 2DH0
rxn2 - DH0rxn3
= 87.3 kJ/mol
2 x-1 x
Experiment Tips
Rinse and reuse styrofoam cups Save cups for next class Do not poke hole in cups with
temperature probe Record exact volumes of each
solution
Experiment Tips
Push collect before mixing solutions Values of DT will vary for each of the
three partsPress “Autoscale” button in LoggerPro
to expand graph Don’t wait until the last minute to
complete calculationsAllow time to get help, if needed
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